(a) Field of the Invention
The present invention relates to a lubrication structure of a camshaft with a variable valve timing.
(b) Description of the Related Art
Generally, in order to enhance an efficiency of an engine, it is required to vary opening/closing timing of valves depending on an engine speed. In particular, opening/closing timing of an intake valve is a factor having a significant effect on a volumetric efficiency. If the intake valve is opened in advance, a valve overlap period becomes long so that intake and exhaust inertia flow can be sufficiently used at a high speed, and accordingly a volumetric efficiency is enhanced, but at a low speed, a volumetric efficiency is deteriorated because of increase of remained gas so that amount of exhausted HC gas is increased.
In order to solve this problem, continuously variable valve timing (CVVT) has been introduced in order to varying opening/closing timing of an intake valve. A continuously variable valve timing assembly includes a hydraulic circuit for advancing or retarding timing of an intake camshaft by hydraulic pressure controlled by an oil pump and an oil-flow control valve.
The continuously variable valve timing assembly is generally connected to a camshaft, and oil delivered from a cylinder block is supplied to a retard flow passage and an advance flow passage through a retard hole and an advance hole which are formed in the camshaft so as to perform lubrication.
However, oil pressure and lubrication at an advanced side at which the advance hole is formed is insufficient at an initial stage of cold starting or an engine idling, and a journal of the camshaft may be damaged by load by a sprocket which is connected to the camshaft. That is, adherence due to insufficient lubrication of the camshaft may occur.